Drill a 1/4-inch hole through the face of a wooden ruler at the 1-inch mark.
Drive a 1/4-inch diameter nail through the top of your 12-inch by 12-inch by 1 ½-inch thick wooden board. Position the nail at the bottom edge in the middle.
Place the ruler over the nail.
Position the head of a pencil next to the 5-inch mark and then rotate the ruler around the nail while you press the pencil against the board. This will give you a perfectly half round line.
Move the pencil to the 6-inch line and then repeat the half circle. This creates the first band of the rainbow. Move the pencil to the 7-inch line to create the second band of the rainbow. Move the pencil to the 8-inch line to create the third band of the rainbow. Move the pencil to the 9-inch line to create the fourth band of the rainbow. Move the pencil to the 10-inch line to create the fifth band of the rainbow. Move the pencil to the 11-inch line to create the sixth band of the rainbow. Move the pencil to the 12-inch line to create the seventh band of the rainbow.
Cut out each band of the rainbow with a band saw. Stay as close to the lines as possible to create a uniform and professional looking rainbow model.
Sand the cut edges of the model with 600-grit sandpaper. Graduate to 800- and then 1000-grit sandpaper to achieve a smooth finish.
Paint the largest band red, and then paint each subsequent band orange, yellow, green, blue, indigo and violet. Use your choice of paints for this. You can spray the band with spray paint or use a brush and acrylic paint. If you plan to let children play with the model, use child-safe paint. Allow adequate drying time.
Stack the bands on top of each other to assemble the rainbow model.